In the current technology of the semiconductor integrated circuit, higher integration has been achieved and as a result, the minimum pattern size reaches the region of 100 nm or less. For the formation of fine patterns, exposure technique is regarded as very important, and the exposure technique enables to attain a desired pattern in the following manner. At first, a resist film is applied onto a substrate to be processed (surface to be processed) to which a thin film has been formed, the resist film is selectively exposed with light and then developed so as to form a pattern, a dry etching is performed using the thus obtained pattern as a mask, and finally the resist pattern is removed to thereby obtain the desired pattern.
In order to realize downsizing of the pattern, it is effective to improve and develop both an exposure light source using the shorten wavelength and a resist material of high resolution corresponding to the characteristics of the exposure light source. Currently, ArF excimer laser exposure tools have been on the market. However, these exposure tools themselves are quite expensive and a large scale of cost is expected at the time the exposure tool is updated for the purpose of shortening the wavelength of the exposure tool. Moreover, it is not easy to develop a resist material which corresponds to the shorten wavelength of exposure light, and it is extremely difficult to realize the downsizing of the pattern by only shortening the wavelength of the exposure device.
For these reasons, attentions have been attracted to a new exposure technique, a liquid immersion exposure method, in the art. In this method, the space between the projection lens and wafer in the exposure device is filled with a liquid having a lager refractive index n than that of air so as to improve and obtain higher resolution than that of the related art.
The resolution of the exposure device is determined by using the following Calculation Formula 1:Resolution R=Coefficient k×Wavelength λ of light source/Numerical aperture NA  Calculation Formula 1
As represented with Calculation Formula 1, the resolution R improves (be smaller), as the wavelength λ of an exposure light source is shorter and the numerical aperture NA is larger. Note that, the numerical aperture of the projection lens is represented as: NA=n×sin α, where n is refractive index of a medium through which the exposure light is transmitted, and α is an angle formed between the exposure light and a light axis of the projection lens. Since the exposure of light is generally performed in atmospheric air, the refractive index n is 1 (i.e., n=1). The liquid immersion exposure method applies the exposure system in which the space between the projection lens and the wafer is filled with a liquid having the refractive index n larger than 1 (i.e., n>1). Accordingly, the refractive index is enlarged from 1 to n (a number larger than 1) in the relative formula of the numerical aperture NA: NA=n×sin α. At the incident angle α of the same exposure light, the resolution R (minimum resolution size) will be reduced in 1/n as NA is enlarged n time(s). In addition, there is also the advantage such that, in the case where the value of NA is set the same, the focal depth is deepened n times as a can be reduced by enlarging n.
The liquid immersion exposure method, which uses the liquid having larger refractive index than that of air, is a known technique in the field of microscopy. However, the development of this method for the application of the fine processing technology has just begun, and the problems have gradually been clear in the course of the development. One of the sever problems is such that the sensitivity of the resist material is lowered as the acid generated within the resist film at the time of the exposure is released (eluted) in liquid, e.g. water, as the resist film is exposed with the water present between the projection lens and the wafer. Moreover, in the case where the excimer laser is transmitted to the resist film in the state where the water is penetrated in the resist film, the inherent characteristics of the resist material is ruined due to the chemical reaction, which would not be caused in the conventional dry atmosphere, or the eluted substance will be a factor to cause the contamination of the lens or the like of the exposure device.
Other than the technique mentioned above, the fine processing technique using a EUV (i.e., extreme ultraviolet ray, wavelength: 13.5 nm) light source has been vigorously developed as the exposure technique for the next generation. Recently, the exposure device called α-demonstration tool has begun to run experimentally. The environment for the development of the resist material for this technique has also been gradually prepared. Accordingly, there is also a demand for a development of a resist material corresponding to the EUV exposure technology which is performed in a highly vacuumed atmosphere. One of the biggest challenges for carrying out the exposure in highly vacuumed atmosphere is to maintain the characteristics, e.g. sensitivity, resolution, less roughness etc., of the resist material, at the same time as reducing the degassing (out gassing) from the resist film. If the amount of the out gassing is large, the contamination is accumulated on the reflective projection optical system, and thus reflectivity of the optical mirror is reduced. In this regard, the development of the resist material which reduces out gassing as much as possible is the urgent need. As a result of recent researches, it has been found and noted that the acid generating agent contained in the chemical amplified resist material is decomposed by the exposure light, and this decomposed product is a source for out gassing which significantly contaminates the mirror.
It has been pointed out the possibility that the source of the contamination for the lens or the like in the process of the liquid immersion exposure is a decomposed product of a cation site, which is produced by the exposures, not the anion site of the acid generating agent which is conventionally determined by LC-MS. Therefore, there has been a demand for the acid generating agent having such structure that the cation site is not easily eluted. Moreover, in the process of the EUV exposure, there is a report which informs that the main factor for contaminating the mirror is considered to be a hydrocarbon substance. Therefore, the structure of the cation site of the acid generating agent is preferably modified in either of the exposure methods.
As mentioned above, the consideration is given to the prevention of contaminating the exposure device or the like due to the main factor of the acid generating agent contained in the resist material itself in both liquid immersion exposure method and EUV exposure method. However, if the content of the acid generating agent is merely reduced, the sensitivity or resolution may be lowered. Therefore, such method may not be acceptable as a solution. There is also an attempt for reducing the aforementioned elution or out gassing by optimizing the structure of the acid generating agent. However, the optimization of the structure and the lithographic performance are difficult to attain at the same time, and thus it may take a long period of time to develop such the acid generating agent.
In order to solve these problems, there has been proposed a method for reducing elution or out gassing, in which an acid generating side chain is introduced into a base resin itself, not adding the acid generating agent as in the conventional manner, and this method has been remarked as a one of the effective solutions (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2004-162040, 2007-161987, and 2007-197718, and U.S. Pat. No. 7,049,044). It has been known that the acid generating side chain is capable of functioning in a small amount thereof compared to the addition of the conventional acid generating agent since the acid generating side chain is closely present to an acid liable group in the polymer side chain, as well as that the acid generating side chains are uniformly present in the resist film. Therefore, such acid generating side chain effectively works for reducing out gassing or elution. However, not so many variations of monomer having such acid generating side chain or resin using the same are known. In addition, most of them contain aromatic rings, and thus there is a problem such that the transparency can be lowered at ArF wavelength. Therefore, there has been a demand for a widened selection of materials, and a development of the material which is easily produced. Furthermore, as mentioned above, it has not been yet known any material having a structure in which a cation site is not easily eluted or out-gassed, together with the structure mentioned above.